Static magnetic apparatuses, for example transformers and inductors, are major elements of circuits which are designed for storage and conversion of energy, for impedance matching, for filtering, for suppression of electromagnetic interference radiation or else for voltage or current conversion. Furthermore, these components are also major components of resonant circuits. Inductive components are based on the production of magnetic alternating fields by primary currents, which themselves induce secondary currents. They can therefore be manufactured for high frequencies with acceptable compactness and efficiency, without magnetic materials, by suitable arrangement of the current paths. For miniaturization, partially planar windings, which can be integrated in conventional multilayer circuit mounts composed of organic or ceramic materials, have been proven over wire-wound, relatively costly components. In this case, in particular, the widely used circuit mounts composed of FR4 material or LTCC (Low Temperature Cofired Ceramic) technology may be mentioned. In this technology, unsintered ceramic green films are provided with vias and planar line structures by stamping and screen printing methods using metal-filled, electrically conductive pastes, and are then sintered together in a stack. This results in substrates which can be thermally loaded, have low losses, are hermetically sealed and can be populated further in a conventional manner.
For the wide field of application of current and voltage transformation, as well as for low-pass filters in power electronic circuits, the low frequencies result in a need for components with better magnetic coupling based on magnetic materials, which can reinforce and shape the magnetic flux. A wide range of variants of coil and transformer cores composed of ferritic ceramic are commercially available for this purpose and can be subsequently attached, with the aid of metal brackets, to the planar circuit mounts that have been mentioned.
It has not yet been possible for completely monolithic solutions, which promise more cost-effective manufacture in a blank, to become established, because of more far-reaching demands relating to material and process technology. One problem aspect in this case is that an increase in the magnetic performance of ferrites, that is to say the permeability of the material, with the aid of ceramic technologies results, from experience, in a decrease in their resistivity and therefore a decrease in the important DC voltage isolation between the primary and secondary sides of the transformer. In order to counteract this, it is in principle possible to embed turns which carry the current in material which provides good insulation and has low permeability. This corresponds to the wire insulation and air in the case of wire-wound components.
The two spatial regions with high magnetic permeability on the one hand and good insulation of the turns on the other hand are illustrated in the basic form in FIG. 1. This figure shows a toroidal core 1, which is ringed on the one hand by a primary winding 2 and on the other hand by a secondary winding 3. FIG. 2 shows a further basic refinement in which two toroidal cores 1a and 1b are provided, which are arranged alongside one another in the horizontal direction, with both toroidal cores 1a and 1b being ringed by a primary winding 2 and a secondary winding 3, which are arranged one on top of the other horizontally.
FIG. 3 shows a section illustration on the plane of the primary winding 2, as shown in the illustration in FIG. 2. The winding 2 is in this case shown by dashed lines and surrounds a central area 11 of the ferrite core, which is formed by the toroidal cores 1a and 1b. The toroidal cores 1a and 1b form a ferrite core of the inductive component. The vertical ferrite limbs which are shown in the section illustration are closed by ferrite covering layers on the upper face and lower face to form these toroidal cores 1a and 1b. The windings 2 and 3 as well as the toroidal cores 1a and 1b are embedded in a dielectric 4.
FIG. 4 shows a further section illustration, illustrating an approximation to a pot-type core with five vertical limbs composed of ferrite material. The limbs are characterized by the central area 11 and the vertical outer limbs 1a, 1b, 1c and 1d. In this case as well, the arrangement is embedded in an insulating dielectric medium.
U.S. Pat. No. 5,349,743 discloses a method for manufacturing a monolithically integrated planar transformer based on LTCC technology. The basic structures shown in FIGS. 1 and 2 are in this case manufactured by connection of a material with low permeability with a relatively high resistivity and of a material with a higher permeability and a lower resistivity. These two materials are integrated by stamping out openings in the films of one material, filling the openings with film pieces or film stacks of the other material, and then sintering them jointly. This inlaying process is complex and susceptible to errors, even with materials which are well matched to one another, and is therefore also relatively expensive, since the films must be processed abutting.
Furthermore, U.S. Pat. No. 6,198,374 discloses a method based on conventional LTCC technology. In this method, just one film type, specifically that composed of the most suitable ferrite, is used in order to print on the conductor tracks. These are then coated, for example by screen printing, with non-magnetic, dielectric material. The aim of this is to reduce the effective permeability and the stray inductance, caused by leakage of field lines, in the vicinity of the turns of a winding. An additional aim is in this way to improve the electrical insulation between the turns. This has the disadvantage of the additional material layer in the area of the turns, which cannot be chosen to be indefinitely thick, in order to avoid stress cracking. In particular, the conductor tracks themselves must actually be made as thick as possible for power-electronic applications, in order to reduce resistance losses. The known method therefore offers only restricted effectiveness.